Ultrasonic welding performs solid-phase welding on two metal plates that have been placed on an anvil in a stacked state by pressing a horn that vibrates against the two metal plates.
In connection with this, in Japanese Patent Laid-Open Publication No. H5-115986, there is proposed an ultrasonic welding monitoring method of measuring a vibration of an anvil in ultrasonic-welding, comparing a measured waveform of vibration with a standard waveform, and determining the quality of ultrasonic welding. According to the monitoring method disclosed in Japanese Patent Laid-Open Publication No. H5-115986, the quality of the welding state of the two ultrasonic-welded metal plates can be determined with ease.
However, in the above-mentioned monitoring method, since the measured waveform of vibration is compared with the standard waveform, if the measured waveform is different from the standard waveform, a product will be determined to be a defective product. Therefore, there is such a problem that even a product that will be determined to be a non-defective product if a tension test is performed will be determined to be a defective product if the measured waveform is different from the standard waveform, and determination accuracy is low.